OKCupid has many fake profiles. Some are real people, just using fake pictures. Some are obvious fakes with copy-paste description that you'd see on hundreds of profiles and professional photoshoot photos that have hundreds of matches with google image search. There is a more advanced fake profile method though. It involves professional scammers writing people lengthy, personalized messages for days or even weeks, and then luring them into payment, either through some different "russian brides" website, or telling some sob story about missing money for rent and asking for help.
I met my previous girlfriend on OkC, with whom I was for 3 years, and the current one, 1 year and counting, hopefully a lot of counting.
There are definitely fewer people than e.g. Tinder, but they tend to be a lot more real vs spammers / scammers / engineered profiles designed to keep you in.
Most Tinder matches were bots, most OkC matches were actual people. Of course this is with a sample size of 1, in Germany. Your mileage may vary.
English is not my first language, but yes, to my knowledge that is grammatically correct.
Maybe it is a bit ambiguous, although from the context it should be reasonably clear that I was with my (now-ex) girlfriend for that period of time, and not with OkC itself :)
Most English speakers would say, "a woman who I was with for 3 years." In this case "with" is understood to mean in a romantic relationship.
I don't think what you said is technically correct. "With whom I was" calls for something (a verb or phrase) to be tacked onto the end, but it would still be awkward. For example, "with whom I was dating for three years."
From another perspective: whom is one of the few relics of the entirely dropped dative case in English (via the also disappearing objective or oblique case which mostly only remains in English personal pronouns I versus me). It's generally relegated only to formal writing, and some (including the Oxford English Dictionary since the 1980s) consider "whom" to be entirely deceased, ancient, and no longer an active English word.
I'd say OKcupid is better than most (the higher barrier to entry i.e. having to answer questions) helps to a degree but there's still plenty of fake / scammer accounts.